Get the height/width from the preset/style configuration before testing the image directly

So, we're caching the height/width of the image as measured with getimagesize() and this makes it faster and avoids system calls but we should be able to get the final dimensions from the preset itself.

The main advantage of this is that we can use it while the image is still being generated. Currently, there is no height or width attributes added to image tags while images are being generated in D6 - reasonably sure this was fixed in D7 though.